Antiproliferative factor (APF) is a sialoglycopeptide inhibitor of bladder epithelial cell proliferation that is at least secreted specifically by bladder epithelial cells from patients with interstitial cystitis (IC) (Keay et al., 2004; Keay et al., 2000) a disorder commonly associated with denudation or thinning of the bladder epithelium (Skoluda et al., 1974; Matthews et al., 2001; Held et al., 1990). APF was discovered to be the active factor in urine from IC patients that reversibly inhibited the growth of bladder epithelial cells in vitro (Keay et al., 2000; Keay et al., 1996). The specificity of APF for urine from IC patients (vs. normal controls or patients with a variety of other urogenital disorders (Keay et al., 2001)) indicates that in certain aspects it is useful as a diagnostic marker for IC and that it may play an important role in the pathogenesis of this disorder.
APF is the first naturally occurring, low molecular weight negative growth regulator to have been identified and completely characterized. The peptide sequence of APF is identical to residues 541-549 of the 6th transmembrane domain of Frizzled 8, a Wnt ligand receptor. The glycosyl moiety of APF comprises sialic acid α-2,3 linked to galactose β1-3-N-acetylgalactosamine, which is α-O-linked to the N-terminal threonine residue of the nonapeptide 1.
APF has been shown to profoundly inhibit the proliferation of both normal bladder epithelial cells and bladder carcinoma cells in vitro (Keay et al., 2004; Keay et al., 2000; Keay et al., 1996). Furthermore, APF can induce multiple changes in the pattern of cellular gene expression including decreased production of heparin-binding epidermal growth factor-like growth factor (HB-EGF) and increased production of E-cadherin, resulting in a more differentiated bladder epithelial cell phenotype (Keay et al., 2000; Keay et al., 2003). APF was also recently determined to decrease tight junction protein (zonula occludens-1 and occludin) production and increase paracellular permeability of normal bladder epithelial cell monolayers similar to changes seen in cells from patients with IC in vitro (Zhang et al., 2005).
The potency of APF (EC50 in the picomolar range), its varied effects on bladder epithelial cell protein expression and proliferation, and the requirement for a hexosamine-galactose disaccharide linked in a specific alpha configuration to the backbone peptide for activity (Keay et al., 2004; Keay et al., 2000; Zhang et al., 2005), all indicate that APF's effects are mediated by binding to and activating a receptor, for example. Microarray analysis indicated that there may be a role for specific transcription factors, such as AP-1, SP-1 and TCF/LEF-1, in abnormal gene expression in cells explanted from IC patients or following APF treatment of normal cells; this provides additional evidence for involvement of a receptor (Keay et al., 2003). Conrads et al. (2006) show that CKAP4/p63 is a receptor for the frizzled-8-protein-related antiproliferative factor from interstitial cystitis patients. Modulation of an APF receptor is useful for therapy/prevention of the effects of APF on the bladder epithelium.